The design and manufacture of vehicle drive systems, such as automotive drive trains, required testing of all drive system components. Among the components suited for testing have been internal combustion engines, transmissions, gears, bearings, axles, clutches, drive shafts, braking systems and combinations of such components. The term combustion engine, in the context of the present invention, should be interpreted to include gasoline engines, diesel engines and fuel-powered turbines. Prior testing devices have attempted to simulate actual running load conditions which would be experienced by the drive system specimens being tested. Realistic testing of components mandated prior testing apparatus to induce torque loads sufficient for examination of acceleration, deceleration and shifting characteristics of the specimens being examined. For such purposes, prior test systems attempted to simulate, as accurately as possible, rotating and linearly moving masses existing in a vehicle.
Among the prior rotating mass simulator test apparatus were test equipment having direct current electric drive motors controlled by electronic regulators. Such test apparatus suffered, however, from several disadvantages. Initially, they were relatively high in cost due to the cost of both the electric motors and the necessary static converters which were required for operability. In addition, because of their large mass, they occupied a considerable amount of floor space and were quite heavy. Further, rapid control of torque variation was not possible due to the high inductance of the motor windings, or, if at all possible, was achieved only at a relatively high cost. Finally, the mass moment of inertia of a typical electric motor armature was in the order of 5 to 25 times greater than the mass moment of inertia of the internal combustion engine being tested. As a result, simulation of small masses such as loads of internal combustion engines was not possible except through the sacrifice of response time and accuracy which, of course, prevented realistic load simulation.
An attempt has been made to provide testing equipment for internal combustion engines wherein, in lieu of direct current electric motors which operated as motors or generators, a hydrostatic pump/motor was employed. A typical system of this type was described in the following publication: Mannesmann Rexroth RV 03065/09.85, .COPYRGT.1985. Such test equipment was more cost effective and structurally simpler than the electric motor test apparatus. Further, the mass moment of inertia of the hydrostatic pump/motor unit was smaller than that of internal combustion engines which were being tested.
In the Rexroth system, illustrated in FIG. 3 infra, an internal combustion engine 1 was connected through a step down gearing 2 to a hydrostatic unit 3. The input speed of the hydrostatic unit 3 was coupled to an accelerator pedal through a proportional valve 4. When the engine was idling, a relatively small swivel plate pivot angle was provided by the valve 4 which resulted in a relatively small hydrostatic fluid feed volume through adjustment of the piston stroke of the hydrostatic unit. The small feed volume was sufficient to support the torque generated by the internal combustion engine at constant operating pressure of the hydrostatic fluid. The volume of hydrostatic fluid, normally oil, depended upon the rotational speed and pivot angle of the hydrostatic unit 3. When the speed increased, the delivered quantity of hydrostatic fluid increased at the same pivot angle. The torque of the engine was determined by the position of its throttle valve.
The hydrostatic unit 3, also known as a secondary unit, reacted to an increase of the engine torque through an increase of the pivot angle with the delivered hydrostatic fluid quantity becoming greater at constant speed input. The operating pressure in the system remained constant during this procedure. Such operating pressure was regulated and adjusted at a second hydrostatic unit 5. The hydrostatic unit 5, when acting as a motor, drove an electric motor 6, such as a 3-phase asynchronous motor. When the motor 6 was driven into supersynchronous speed range, it then operated as a generator and fed current back into the current supply.
When the system operated in simulation of downhill runs, the hydrostatic unit 5 operated as a pump while the first hydrostatic unit 3, i.e. the secondary unit, operated as a motor for driving the engine 1.
Among the problems encountered with such hydrostatic test system was that it was unable to realistically simulate small mass moments of inertia while providing precise regulation and adjustment of desired loads. The mass load of the system was limited by the total mass moment of inertia which could be generated at the secondary unit. Further, such system was unable to provide rapid torque load variation encountered during actual usage of the components being tested.